1. Field of the Invention
The present invention relates to an electrolyte for a lithium ion rechargeable battery and a lithium ion rechargeable battery that comprises the electrolyte. In particular, the present invention relates to an electrolyte for a lithium ion rechargeable battery that improves the overcharge stability of a battery and provides excellent electrochemical properties and a lithium ion rechargeable battery comprising the same electrolyte. These electrochemical properties include reducing the swelling of the battery, high-temperature storage stability and cycle life characteristics.
2. Description of the Related Art
As portable electronic devices are rapidly becoming smaller and lighter, the batteries that are used to power them are increasingly required to have a compact size and a large capacity. For example, a lithium ion rechargeable battery has a drive voltage of 3.6 V or more, which is three times higher than the drive voltage of a nickel-cadmium (Ni—Cd) battery or nickel-metal hydride (Ni—MH) battery that is currently used as a power source for a portable electronic device. Further, a lithium ion rechargeable battery has relatively high energy density per unit mass. Therefore, research and development into lithium ion rechargeable batteries are increasing and progressing rapidly.
A lithium ion rechargeable battery comprises a cathode including a lithium-containing metal oxide, an anode including a carbonaceous material that is capable of lithium intercalation/deintercalation, and a non-aqueous electrolyte solution that contains an electrolyte in a non-aqueous solvent.
Lithium rechargeable batteries use a non-aqueous electrolyte because of strong interactions between lithium and water. Such an electrolyte may include a lithium salt-containing solid polymer or a liquid electrolyte containing a lithium salt that is dissociated in an organic solvent. Generally, the organic solvent in which a lithium salt is dissolved may be ethylene carbonate, propylene carbonate, other alkyl group-containing carbonates, and the like. These solvents have boiling points of 50° C. or higher and very low vapor pressure at room temperature.
Lithium rechargeable batteries are classified into two groups including lithium metal batteries and lithium ion batteries that use liquid electrolytes and lithium ion polymer batteries that use solid polymer electrolytes. Lithium ion polymer batteries are further classified into full solid type lithium ion polymer batteries that contain no organic electrolyte solution and lithium ion polymer batteries that use gel type polymer electrolytes that contain an organic electrolyte solution.
When a lithium ion rechargeable battery is overcharged or it experiences an electric short, thermal runaway, during which the battery temperature increases rapidly, may occur. An overcharge or electric short may result from the misuse of a battery or malfunction of a charger, etc. This damage to the battery causes an excessive amount of lithium to be discharged from a cathode and precipitated onto the surface of an anode, thus placing both electrodes in a thermally unstable state. This thermal instability causes thermal decomposition of an electrolyte through reactions between an electrolyte and lithium, oxidation of an electrolyte at a cathode, reactions between oxygen generated from thermal decomposition of a cathode active material and an electrolyte, and the like. These exothermic interactions may cause a rapid increase in battery temperature, or thermal runaway. If the temperature of a battery exceeds a maximum acceptable temperature, an explosion or emission of smoke from the battery may occur.
Many attempts have been made to solve these problems by including various additives in a non-aqueous electrolyte.
Japanese Laid-Open Patent No. Hei 9-50822 discloses a method for ensuring overcharge stability of a battery by adding a benzene compound that comprises a desired substituent, such as an anisole derivative, to a non-aqueous electrolyte of a rechargeable battery. The anisole derivative acts as a redox shuttle in an overcharged battery. As such additives provide good reversibility of redox reactions, they also consume extra electric current caused by overcharge while running between a cathode and an anode.
In addition, Japanese Laid-Open Patent No. Hei 11-162512 discloses a method for ensuring stability of a battery under overcharged conditions by adding a small amount of aromatic compounds such as biphenyl, 3-chlorothiopen, furan, etc., and electrochemically polymerizing them in an abnormal over-voltage state to increase the internal resistance. However, when these aromatic additives are used at temperatures greater than 40° C. or when a relatively high voltage is generated locally during their use under a normal drive voltage, they may decompose gradually during repeated charge/discharge cycles, which results in deterioration of battery characteristics.